662 GASTRO-INTESTINAL BACTERIOLOGY 
fairly constant in their occurrence, but there may be a well-marked 
seasonal and even annual variation in the relative proportions of the 
individual groups of organisms which comprise this flora. This sug- 
gests that the normal bacterial flora is acclimatized to the intestinal 
environmental conditions of temperature, reaction and composition 
of food, and of intestinal secretions at different levels. It also indi- 
cates that the activities of the organisms which comprise the normal 
intestinal flora are not in active opposition to those of the host.^ 
Adventitious bacteria, frequently in considerable numbers, un- 
doubtedly reach the intestinal tract from time to time. The fate 
of these organisms depends upon a number of factors, some of which 
are little understood. If their activities are greatly at variance with 
those of the normal types they usually fail to gain a foothold; either 
they are unable to develop in competition with the well-acclimatized 
normal flora, or they cannot accommodate themselves to the physio- 
logical and chemical conditions which prevail there. If, on the con- 
trary, these organisms can adapt themselves readily to the prevailing 
conditions at some level of the alimentary canal they may continue 
to develop either in association with preexisting types, or gradually 
replace the latter.' It is doubtless through this process that the sea- 
sonal prevalence of some types of intestinal bacteria has its origin. 
It is not unlikely, furthermore, that the occasional unusual type of 
organism characteristic for an individual or a group of individuals 
gains entrance to and develops in the intestinal tract in this manner. 
The nature of the process whereby progressively pathogenic bacteria 
(usually of exogenous origin) replace or modify the normal intestinal 
flora is as yet little understood. There is evidence in favor of the view 
that exogenous bacteria which invade the body through the intestinal 
tract may become somewhat widely disseminated in restricted areas 
and appear in the intestinal contents of many individuals without 
inciting noteworthy symptoms, prior to the appearance of disease in 
epidemic proportions^ and with characteristic symptoms. 
1 The general phenomena governing the parasitism of bacteria in the alimentary 
canal are not unlike those leading to bacterial parasitism upon the skin, the conjunctiva, 
or other surfaces of the body which are in communication with the exterior. One 
important phase of intestinal parasitism is not manifested in other parts of the body 
however. The bacteria of the intestinal flora change along rather definite lines from 
infancy to adult life, as the diet of the host changes from the monotonous pabulum of 
infancy to the varied regimen of the adult. The organisms parasitic upon the skin and 
other surfaces of the body do not exhibit this change in tj-'pe, and it is reasonable to 
attribute the relative stability of the skin flora to the relative constancy of environ- 
mental conditions there, while the succession of t>i3es of intestinal bacteria from infancy 
to adult life is rather definitely associated with corresponding changes in the diet of the 
host. 
2 Undoubtedly repeated inoculation of the alimentary canal with adventitious strains 
of bacteria plays an important part in determining their acclimatization in the intestines; 
possibly a simultaneous absence of the preexisting intestinal types in the environment, 
leading to a reduction or even absence of these normal inhabitants in the food of the 
host may materially affect the outcome of the "replacement" process. 
3 Kendall: Boston Med. and Surg. Jour., 1915. 172, 851. 
